(1) Field of the Invention
The present invention relates to a method and apparatus for feedback control of an air-fuel ratio in an internal combustion engine having two air-fuel ratio sensors upstream and downstream of a catalyst converter disposed within an exhaust gas passage.
(2) Description of the Related Art
Generally, in a feedback control of the air-fuel ratio sensor (O.sub.2 sensor) system, a base fuel amount TAUP is calculated in accordance with the detected intake air amount and detected engine speed, and the base fuel amount TAUP is corrected by an air-fuel ratio correction coefficient FAF which is calculated in accordance with the output of an air-fuel ratio sensor (for example, an O.sub.2 sensor) for detecting the concentration of a specific component such as the oxygen component in the exhaust gas. Thus, an actual fuel amount is controlled in accordance with the corrected fuel amount. The above-mentioned process is repeated so that the air-fuel ratio of the engine is brought close to a stoichiometric air-fuel ratio.
According to this feedback control, the center of the controlled air-fuel ratio can be within a very small range of air-fuel ratios around the stoichiometric ratio required for three-way reducing and oxidizing catalysts (catalyst converter) which can remove three pollutants CO, HC, and NO.sub.X simultaneously from the exhaust gas.
In the above-mentioned O.sub.2 sensor system where the O.sub.2 sensor is disposed at a location near the concentration portion of an exhaust manifold, i.e., upstream of the catalyst converter, the accuracy of the controlled air-fuel ratio is affected by individual differences in the characteristics of the parts of the engine, such as the O.sub.2 sensor, the fuel injection valves, the exhaust gas recirculation (EGR) valve, the valve lifters, individual changes due to the aging of these parts, environmental changes, and the like. That is, if the characteristics of the O.sub.2 sensor fluctuate, or if the uniformity of the exhaust gas fluctuates, the accuracy of the air-fuel ratio feedback correction amount FAF is also fluctuated, thereby causing fluctuations in the controlled air-fuel ratio.
To compensate for the fluctuation of the controlled air-fuel ratio, double O.sub.2 sensor systems have been suggested (see: U.S. Pat. Nos. 3,939,654, 4,027,477, 4,130,095, 4,235,204). In a double O.sub.2 sensor system, another O.sub.2 sensor is provided downstream of the catalyst converter, and thus an air-fuel ratio control operation is carried out by the downstream-side O.sub.2 sensor is addition to an air-fuel ratio control operation carried out by the upstream-side O.sub.2 sensor. In the double O.sub.2 sensor system, although the downstream-side O.sub.2 sensor has lower response speed characteristics when compared with the upstream-side O.sub.2 sensor, the downstream-side O.sub.2 sensor has an advantage in that the output fluctuation characteristics are small when compared with those of the upstream-side O.sub.2 sensor, for the following reasons:
(1) On the downstream side of the catalyst converter, the temperature of the exhaust gas is low, so that the downstream-side O.sub.2 sensor is not affected by a high temperature exhaust gas.
(2) On the downstream side of the catalyst converter, although various kinds of pollutants are trapped in the catalsyt converter, these pollutants have little affect on the downstream side O.sub.2 sensor.
(3) On the downstream side of the catalyst converter, the exhaust gas is mixed so that the concentration of oxygen in the exhaust gas is approximately in an equilibrium state.
Therefore, according to the double O.sub.2 sensor system, the fluctuation of the output of the upstream-side O.sub.2 sensor is compensated for by a feedback control using the output of the downstream-side O.sub.2 sensor. Actually, as illustrated in FIG. 1, in the worst case, the deterioration of the output characteristics of the O.sub.2 sensor in a single O.sub.2 sensor system directly effects a deterioration in the emmision characteristics. On the other hand, in a double O.sub.2 sensor system, even when the output characteristics of the upstream-side O.sub.2 sensor are deteriorated, the emission characteristics are not deteriorated. That is, in a double O.sub.2 sensor system, even if only the output characteristics of the downstream-side O.sub.2 are stable, good emission characteristics are still obtained.
In the above-mentioned double O.sub.2 sensor system, for example, an air-fuel ratio feedback control parameter such as a rich skip amount RSR and/or a lean skip amount RSL is calculated in accordance with the output of the downstream-side O.sub.2 sensor, and an air-fuel ratio correction amount FAF is calculated in accordance with the output of the upstream-side O.sub.2 sensor and the air-fuel ratio feedback control parameter. In this case, the air-fuel ratio feedback control parameter is stored in a backup random access memory (RAM). Therefore, when the downstream-side O.sub.2 sensor is brought to a non-activation state or the like to stop the calculation of the air-fuel ratio feedback control parameter by the downstream-side O.sub.2 sensor, the air-fuel ratio correction amount FAF is calculated in accordance with the output of the upstream-side O.sub.2 sensor and the air-fuel ratio feedback control parameter stored in the backup RAM (see Japanese Unexamined Patent Publication (Kokai) No. 61-192828, which corresponds to Japanese Patent Application No. 60-32863, which in turn corresponds to U.S. patent application Ser. No. 831,566 filed Feb. 21, 1986, Further, when the air-fuel ratio feedback control parameter is too large or too small, the transient characteristics of the parameter are reduced, or the drivability is deteriorated by the fluctuation of the air-fuel ratio. To avoiding this, the air-fuel ratio feedback parameter is guarded within a predetermined range defined by a maximum value and a minimum value (see Japanese Unexamined Patent Publication No. 61-234241, corresponding to Japanese Patent Application No. 60-74439, which in turn corresponds to U.S. patent application No. 848,580 filed Apr. 7, 1986.
In an internal combustion engine having an evaporative emission control system, however, when the temperature within a fuel tank is relatively high so that a large amount of fuel vapor is generated therein, such vapor is drawn into a combustion chamber of the engine through a canister by a purge system, and thus the air-fuel ratio becomes too rich. When this state continues, the air-fuel ratio correction amount FAF is inclined to the lean side by the output of the upstream-side O.sub.2 sensor and is held at a minimum value such as 0.8 by which the controlled air-fuel ratio is prevented from becoming overlean. Also, in this case, since it is impossible to completely control the air-fuel ratio by the air-fuel ratio correction amount FAF ,the air-fuel ratio feedback control parameter is also inclined to the lean side. In this state, it is assumed that the engine is stopped and then restarted, to reach a normal driving state. As a result, since the air-fuel ratio correction amount FAF is calculated by using the lean-inclined air-fuel ratio feedback control parameters stored in the backup RAM, the speed of increase of the air-fuel ratio correction amount FAF is relatively small. In addition, since the temperature within the fuel tank is relatively low because the engine has been stopped, so that only a little fuel vapor remains in the fuel tank, the output of the downstream-side O.sub.2 sensor maintains a lean signal for a certain period, thus increasing the NO.sub.X emission and reducing the drivability.
Thus, when the air-fuel ratio correction amount FAF is held at the maximum or minimum value of a range by the evaporative emission control system or the like, it is basically impossible for a double O.sub.2 sensor system to detect a deviation of a mean value of the controlled air-fuel ratio, compared with a desired air-fuel ratio such as a stoichiometric air-fuel ratio, and obtain same. Further, if the determination of whether or not the downstream-side O.sub.2 sensor is activated is dependent upon whether or not the output thereof is once changed from the lean side to the rich side, or vice versa, the feedback control for the air-fuel ratio feedback control parameter by the output of the downstream-side O.sub.2 sensor is delayed, thus increasing the exhaust gas emissions, reducing the drivability, and deteriorating the fuel consumption characteristics.